A process of removing semi-solid material from water, often referred to as waste water treatment, is used to treat waste water from manufacturing processes, sewage and the like. To that end, a variety of techniques have been developed and used in the treatment of waste water. One popular technique incorporates a settling tank (alternately referred to as a clarifier) that separates semi-solid material (popularly known as sludge) from the water via gravity. Using this technique, the waste water (influent) enters a clarifier, wherein the sludge is encouraged to settle to the bottom. As the sludge settles, density gradient layers form in the clarifier, with the densest layers disposed toward the bottom of the clarifier. Relatively clear water then exits from the top of the clarifier, while the sludge is removed from the bottom.
Controlling the level of sludge is a key aspect in the effective operation of the clarifier. On the one hand, if the sludge level is too high, contaminated water may exit from the top of the clarifier. On the other hand, if the sludge level is too low, the sludge removed from the bottom of the tank will contain too much water, increasing disposal costs.
Consistently determining the location of a "sludge blanket"--an industry term used to refer to a demarcation level in the clarifier--is the key aspect to controlling the sludge level within the clarifier. The material above the sludge blanket is, in theory, mostly liquid and is more clear, whereas, the material below the sludge blanket is, in theory, mostly solid and is relatively dense. Although the name implies that the sludge blanket location may be readily determined, in practice the determination is less than precise. The challenge in determining the sludge blanket location arises from the nature of the density gradients in the clarifier. That is, the material in the tank is thinnest near the top and densest near the bottom; however, there is no absolute demarcation point where the clear water ends and the sludge begins. Consequently, the techniques currently employed to measure the sludge blanket level provide inconsistent results. This inconsistency leads to inefficient treatment of waste water.
Although determining a sludge blanket level is a significant aspect of waste water treatment, where and how that level is maintained presents trade-offs for the waste water treatment plant operator. One of the most significant trade-offs concerns determining at what level to maintain the sludge blanket. On the one hand, maintaining a high sludge blanket level generally increases the density of the sludge in discarded waste water and results in reduced disposal costs. The disposal costs are reduced because the sludge within the sludge blanket is disposed of when it accumulates to a predetermined level in the bottom of the tank. The disposal cost of the sludge is directly influenced by the percentage of solid material in the discarded waste. The denser the sludge, the more economical the disposal and vice-versa. As a result, disposal costs are decreased because less excess water is transported and disposed with the sludge. As a rule of thumb, sludge blanket densities typically range from 1 to 5 percent solids. If the sludge blanket is too low or non-existent, the sludge removed from the tank will be about 99 percent water. If a high sludge blanket level in the clarifier is high the underlying sludge is generally denser. Thus, there is a strong economic incentive to maintain a high sludge blanket.
On the other hand, current systems that increase sludge density by maintaining a high sludge blanket level have the unfortunate side-effect of increasing the likelihood that contaminated water will exit the tank. That is, as the sludge level rises, the likelihood increases that the water exiting the clarifier will be less clear. So as a trade-off to maintaining a high sludge blanket, closer scrutiny of the clarifier is required because of the potential for short-circuiting the tank, wherein suspended solids do not settle but rather exit out of the top of the clarifier. Such short circuits generally result in downstream pollution and can be the basis for violations of governmental pollution regulations. Thus, there is a need for a system that allows the sludge level to be maintained at a high level, while having an accurate and economical monitoring system.
Several methods have been employed at waste water treatment plants to monitor the sludge blanket level. Among the most widely used--and the most primitive--is a "sludge judge." A sludge judge is a tube that takes a core sample of the clarifier. In operation, the sludge judge is slowly lowered into the water allowing a representative core sample of the water to enter the tube. When the sludge judge has reached the bottom of the tank, the tube is closed and removed from the tank. The translucent tube is then visually inspected and the operator makes a subjective determination of the location of the sludge blanket. The problems with such a technique for monitoring the sludge blanket are numerous and, perhaps, obvious. For example, errors are introduced if the tube is not lowered at the proper rate or angle. Additionally, different sludge blanket determinations will result from taking the core sample at different locations in the tank or from different operators making the determination, which leads to a variable and subjective sludge blanket determinations.
Other devices use portable sonic or optical sensors to determine the sludge blanket location. U.S. Pat. No. 4,940,902 issued to Mechalas et al. discloses such a device. The device consists of a transmitter and a receiver pair (either sound or light), which is lowered into the tank. As the density of the waste water increases, the operator monitors the lowering of the device and the corresponding density either audibly, via a meter or via some other indicator. Although, such devices overcome some of the problems encountered in the use of a sludge judge, additional problems arise. For example, the device must physically enter the sludge blanket. This could agitate the sludge sediment and lead to a false reading. Moreover, as with the sludge judge, different operators may obtain different readings through human error.
In U.S. Pat. No. 4,121,094 issued to DiVito et al., a technique is described for using ultrasonic energy to measure the sludge blanket level. According to the DiVito patent, a transducer is mounted near the top of the tank. The transducer is capable of both transmitting and receiving an ultrasonic signal which is projected toward the sludge blanket. The signal reflected from the sludge blanket is received by the transducer and converted to an electrical signal. However, the DiVito technique is not sufficiently accurate. For example, the sludge blanket is detected by comparing the electrical signal received from the ultrasonic echo to a reference voltage. When the amplitude of the echo signal matches the reference voltage, the DiVito system interprets this as the sludge blanket having reached the maximum height. Pumps are then operated to lower the sludge blanket level. Such a technique may falsely detect transient conditions as a sludge blanket level. For example, sludge disturbed by the skimmer arm could cause the pump to falsely operate. Moreover, thin layers of sludge could bypass detection and exit the top of the clarifier.
All of the techniques described above suffer from the inability to accurately and consistently maintain sludge levels in a clarifier. Applicants have recognized that method and apparatus that could increase the density of disposal sludge while preventing sludge from short-circuiting the tank would greatly enhance the efficiency of waste water treatment plants. Thus, there is a long-felt need for method and apparatus that accurately and consistently maintain the sludge levels within a clarifier.